An excavator is an example of a construction machine that uses multiple hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to a pump that provides pressurized fluid to chambers within the actuators. This pressurized fluid force acting on the actuator surface causes movement of actuators and connected work tools. Once the hydraulic energy is utilized, pressurized fluid is drained from the chambers to return to a low pressure reservoir. Usually the fluid being drained is at a higher pressure than the pressure in the reservoir and hence this remaining energy is wasted once it enters the reservoir. This wasted energy reduces the efficiency of the entire hydraulic system over a course of machine duty cycle.
A prime example of energy loss in an excavator is its swing drive where the fluid emptying to the low pressure reservoir is throttled over a valve during the retardation portion of its motion to effect braking of swing motion. It is estimated that total duration of swing use in an excavator is about 50%-70% of an entire life cycle and it consumes 25%-40% of the energy that engine provides. Another undesirable effect of fluid throttling is heating of the hydraulic fluid which results in increased cooling requirement and cost.